This invention relates generally to indirect rapid cooling, known as quenching, of chemical reactor effluent, for example; and more particularly it concerns improvements in feedstock flexibility to the reactor, as respects yield of primary reaction products, on stream time, and reduction of investment (i.e. simpler cooling apparatus).
Specially designed heat exchangers are used in close couple to the reactor outlet tube. They are known as the transferline exchanger (TLE or TLX), quench exchanger, quench cooler, or quench boiler. Both shell and tube, and double pipe heat exchangers have been used. In this regard, in many chemical reactions, quenching of reactor effluent is required to control the reactions, so that primary products will not undergo secondary reactions. Secondary reactions will not only reduce the yield of valuable primary products but also produce coke which fouls the TLE. Fouling of the TLE has the following effects:
1. Increase in reactor system pressure; thus, yield of primary products decreases with higher system pressure. PA1 2. Energy recovery from the reactor effluent (as for example, heat energy) will decrease. PA1 2. The transition between the reactor outlet tube and TLE inlet tube must have a stream line flow to minimize back mixing which promotes coke formation and fouling. PA1 3. The on stream time must be long, so that the loss of production due to shut-down for decoking is small. PA1 4. The TLE must have a low initial pressure drop, and furthermore, coking must have little effect on the pressure drop. PA1 5. TLE maintenance must be easy and simple. PA1 6. The TLE must operate to maximize energy recovery. PA1 7. The TLE must have feedstock flexibility. PA1 8. The TLE must be cost effective. PA1 9. The TLE must be compact and easy to install. PA1 1. The first tube-pass (inlet pass) consists of a single tube which is connected to the reactor outlet tube of the same diameter; PA1 2. The second tube-pass consists of also a single tube which has a diameter equal to or larger than the inlet tube (the first tube). This tube can be used as the third-pass (outlet pass), depending on the feedstock; and PA1 3. The third tube-pass (outlet pass) consists of several small diameter tubes. These tubes can be used in the second-pass, also depending on the feedstock. The number of the small tubes used can be changed by plugging and unplugging of tubes from both ends. The small tube diameter is less than that of the first tube. PA1 1. The number of tube passes can be varied from one, to two, to three; PA1 2. The length of tube pass of the single tube can be varied from one to two tubes; and PA1 3. The number of operating tubes with small diameter can be adjusted.
When fouling is excessive, the TLE is shut down for cleaning, known as a decoking operation. Since the TLE is coupled with the reactor, the latter will also be shut down. When the reactor is shut down, the production and revenue are reduced, at great expense.
In chemical manufacturing processes, feedstock flexibility is very important. The process must be able to operate on many hydrocarbon feedstocks which are available on the market. To increase feedstock flexibility of a chemical process, more equipment and complicated design of same are required. For example, in olefins manufacturing, ethylene can be made from almost any petroleum products, such as ethane, propane, butanes, naphtha, diesel, kerosene, gas oil, etc. Ethylene is made by pyrolysis of these hydrocarbons in the presence of steam in a tubular reactor. The TLE is then used to quench the reactor effluent. Typical operating conditions of the TLE are as follows:
______________________________________ Shell Side Tube Side ______________________________________ Pressure, PSIG 400 to 2,000 0 to 50 Temperature, degrees F. 440 to 640 1300 to 1800 ______________________________________
In the above, the tube side conditions are at inlet; the pressure drop through the tube side is 0.3 to 2.0 PSI; and the tube side outlet temperature depends upon the feedstock used in the reactor and the shell side quenching medium. These large temperature and pressure differences present problems in design and construction of the TLE. In the quenching process where the temperature of the reactor effluent is reduced, its thermal energy is transferred into the quenching medium, such as vaporization of boiler feed water into steam, superheating steam, or preheating feed.
The design requirements of a TLE are as follows:
1. The residence time in the transferline between the reactor outlet and the TLE inlet must be kept short. Short residence time retards the degradation of primary products.
A practical solution to satisfy these objectives, prior to the present invention was not known.